Japanese Unexamined Patent Application Publication No. 2012-87889 discloses an electric linear motion actuator driven by an electric motor, and configured such that the rotation of the rotor shaft of the electric motor is reduced in speed by a reduction gear mechanism and input to a rotary shaft. The rotation of the rotary shaft is converted by a rotation-to-linear motion converting mechanism to the linear motion of a sliding member axially movable along the inner diameter surface of a housing, thereby axially moving the sliding member.
A plurality of engagement holes are formed in one side surface of one gear of the reduction gear mechanism, while spaced apart from each other on one pitch circle having the center axis about which the one gear rotates. When a locking pin movable toward and away from one point on the one pitch circle is advanced by a linear solenoid until the locking pin engages in one of the engagement holes, the sliding member is locked at an arbitrary axial position.
By using the electric linear motion actuator configured as described above in an electromechanical brake system, when the vehicle is parked, with a brake pad advanced by the sliding member pressing a disk rotor with a predetermined pressing force, the brake pad can be locked such that the vehicle is kept in a stationary state. In this way, such an electromechanical brake system can have a parking brake function.
In the electric linear motion actuator disclosed in JP Publication No. 2012-87889 noted above, it takes a lot of time and costs a lot to form engagement holes in the one gear by means of cutting. By forming the one gear by sintering a metal, costs can be reduced, but grease used to lubricate the reduction gear mechanism is likely to go into the engagement holes. If grease goes into the engagement holes, due to pumping action, i.e. the forward and backward movements of the plunger of the linear solenoid, the grease might be sucked into the linear solenoid, thereby causing the function of the linear solenoid to deteriorate.
In order to overcome this problem, by forming engagement holes comprising recesses, it is possible to prevent grease from going into the engagement holes, and thus to prevent the function of the linear solenoid from deteriorating. Also, by forming the one gear by sintering a metal, costs can be reduced. However, recesses formed when forming the one gear of JP Publication No. 2012-87889 by sintering a metal are not sufficiently deep, i.e., only about several millimeters deep at the most, so that it is impossible to ensure a sufficient length of engagement between the locking pin and such shallow recesses. Therefore, it is impossible to form the one gear by sintering a metal, and to reduce costs while preventing the function of the linear solenoid from deteriorating.